1. Field of the Invention
The present invention relates to an image reading apparatus, an image forming apparatus and an automatic document conveying apparatus. In particular, the present invention relates to a sheet-through type automatic document conveying apparatus, as well as an image forming apparatus and an image reading apparatus having the automatic document conveying apparatus mounted thereon.
2. Description of the Related Art
The sheet-through type refers to a type in which an image reading unit at rest reads a surface of a document conveyed by a conveying mechanism. A sheet-through type image reading apparatus has advantages of size reduction, low cost, low noise, and high productivity, as compared with a platen-set type image reading apparatus whose image reading unit moves and reads a document at rest.
In the case of the sheet-through type image reading apparatus, an image reading position is fixed in place, that is, onto a transparent member (elongated reading glass) and a focal point of a reading optical system is obtained on an image surface of the conveyed document through the reading glass.
As a configuration for reading both the front surface and the rear surface of a document by means of the image reading apparatus, a configuration having a mechanism for turning over a document in the apparatus, thereby turning over the document whose front surface has been read and conveying the document again to an image reading position to read the rear surface has been conventionally common. This manner of reading is referred to as single scan reading.
In contrast, demand for an image reading apparatus further including a reading unit and configured to read the front surface and the rear surface of a document at the same time according to the sheet-through scheme has been expanding rapidly in recent years. This manner of reading is referred to as dual scan reading. The dual scan reading allows double-sided reading with the shortest path. Therefore, as compared with the single scan reading, the dual scan reading has advantages including high reliability and reduction in failed conveyance such as a paper jam, in addition to high productivity and low noise.
In the dual scan reading, a lightweight and compact CIS (Contact Image Sensor) is commonly used. The CIS has a feature of shallower depth of focus than that of a CCD (Charge Coupled Device).
A reading mechanism is generally susceptible to a foreign substance such as dust adhered onto the reading glass and dust remaining on the reading glass. A portion where the foreign substance block light leads to an image noise streak in the read image. In the case of a paper document, a trouble that a minute foreign substance such as a fiber and a filler such as calcium carbonate included in the paper adheres to the reading glass cannot be avoided. In particular, in the case of the CIS having a shallow depth of focus, the document must be brought as close to the CIS as possible. Therefore, an adhesive foreign substance (an adhesive substance protruding from an end face of a portion bonded by tape or glue, correction fluid, an ink lump of a pen, an eraser and the like) adhered to the document are easily transferred to the CIS. The adhesive foreign substance transferred and adhered to the CIS is not easily separated.
As a technique of addressing this problem, Japanese Laid-Open Patent Publication No. 2004-048184, for example, discloses a technique in which a rotating body having a cleaning member is provided at a position where the rotating body faces a CIS, and the rotating body is rotated, thereby cleaning a surface of the CIS.
When such a configuration that the CCD reads the front surface of the document and the CIS reads the rear surface is used in the dual scan reading, however, ensuring the equal image quality in both reading mechanisms may be required.